Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine which includes a rotor for driving the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbofan engines, and turboprop engines.
A recent improvement over the turbofan and turboprop engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 437,923 (abandoned). In the unducted fan engine, the power turbine includes counterrotating rotors and turbine blades which drive counterrotating unducted fan blades radially located with respect to the power turbine.
Conventional pitch changing mechanisms are generally unsuited to the unique mechanical arrangement of the unducted fan engine. More specific concerns include synchronizing the pitch change of each blade on a given blade row, reducing distortions in the event that a blade is lost, and providing balanced synchronization between blade rows.